<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>

圆的半径：<input type="text" placeholder="半径" id="a">
<p></p>
圆的周长：<input type="text" placeholder="周长" id="b">
<p></p>
圆的面积：<input type="text" placeholder="面积" id="c">

<script>
    
    var p = Math.PI;
   var r = prompt("请输入半径");
    r = parseInt(r);

    document.querySelector("#a").value = r;
    document.querySelector("#b").value = 2*p*r;
    document.querySelector("#c").value = p*r*r;


</script>

</body>
</html>